1. Field of the Invention
The present invention relates generally to packaging of fiber optic components, and particularly to methods and apparatuses for packaging long-period fiber gratings, filters, and other fiber optic components to provide support and protection.
2. Technical Background
Long-period fiber gratings are formed by the addition of gratings to a length of optical fiber. Such gratings have an index of modulation along the waveguiding axis of the fiber, and may be formed by writing with UV radiation, etching, or other means of making periodic perturbations. One function of long-period fiber gratings is to couple light between the fundamental mode propagating in the waveguide core and a guided cladding mode.
For high performance applications such as gain-flattening filters (GFFs) in optical fiber amplifiers, long-period fiber gratings must operate over large temperature ranges with minimal change in spectral properties. While the peak loss of the grating will change with temperature, the primary effect of a temperature change is a shift in peak wavelength. Previously, this temperature dependence has been minimized by a variety of techniques including fiber design, fiber composition, and coating material. By varying fiber and grating parameters, both positive and negative wavelength shifts with increasing temperature are possible. The packaging of the optical fiber can compensate for this temperature dependence by attaching the long-period fiber grating to a negative or positive thermal expansion substrate. The packaging is therefore strongly dependent on the characteristics of the long-period fiber grating, which can be tailored to have a variety of strain and temperature dependencies.
Since long-period fiber gratings operate by coupling light between core and cladding modes, they are very sensitive to external perturbations. The grating is typically left uncoated because coatings change the optical properties of the grating. The long-period fiber grating package must therefore protect the region of fiber containing the grating. Some type of tube or rectangular box is therefore desirable to protect the bare fiber from moisture or physical damage, and prevent premature failure. Since long-period fiber gratings are sensitive to bending, the fiber is normally kept relatively straight within the package.
To obtain a typical hermetic (sealed against air and moisture) packaging of a long-period fiber grating, the fiber is metalized and soldered to a high quality package, such as an expensive Kovar.RTM. metal box. The package is then usually attached to a supporting substrate or fixture in a separate step. This solution is expensive both in terms of materials and processing time.
Accordingly, it would be highly advantageous to combine both the fiber support and protective functions in a single package that should protect the fiber from physical deformation as well as protect it from various environmental conditions. The process in which the package is constructed must not impart excessive thermal load to the grating area or damage the optical fiber at the point of contact between the package and the optical fiber.